126P A randomized controlled trial (RCT) comparing optimal local treatment with or without endocrine therapy (ET) in patients (pts) aged ≥70 with lower risk breast cancer (BC) [REaCT-70 Study]

Many RCTs and cohort studies have shown that adjuvant radiation therapy (RT) can safely be omitted in older pts with lower-risk hormone receptor-positive (HR+) BC treated with breast-conserving surgery (BCS) and 5 years of ET. These studies were performed before the widespread use of shorter durations of RT. As patient and healthcare provider surveys suggest a preference for RT over 5 years of ET, we conducted a pilot RCT evaluating the benefits and risks of ET in this population. Eligible pts aged ≥ 70 with lower risk HR+ BC who have had optimal local therapy (i.e. BCS plus RT or mastectomy) were randomized (1:1) to omission or administration of ET for 5 years. The primary endpoint was a combination of feasibility metrics: (1) accrual of at least 100 pts at 8 centers within 2 years, (2) participation rate of at least 60% among pts approached and (3) at least 90% of enrolled pts received treatment as per their allocated arm. Secondary endpoints included adverse events, quality of life and ET discontinuation rates. Reasons for declining study participation were also collected. Between September 2021 and January 2024, the REaCT-70 trial opened at 7 sites. Among the 211 pts approached, 108 pts (50%) agreed to participate. 51 (47%) were randomized to omission and 57 (53%) to administration of ET. Among randomized patients, 7 withdrew consent and 3 were deemed ineligible. Median age was 74 (range 70-93), 16 (16.3%) had a mastectomy and 82 (83.7%) had BCS. In the ET group, 18 (34.6%) received tamoxifen, 30 (57.7%) received an aromatase inhibitor and 4 (7.7%) declined ET after randomization. The main reasons for declining study participation (n=103) were: not wanting ET (63/103, 61%) or wanting ET (16/103, 15%). There were no differences in age, comorbidities, tumor size, surgery type and distance from cancer centre between those who agreed and declined to participate in this study. The REaCT-70 pilot study sparked great interest but the a priori feasibility outcomes were not met. This was mainly because many pts, once aware of their low risk of recurrence, did not want ET. Alternative trial designs are needed to answer this pivotal question.